The present invention relates to an integrated circuit wherein the effective digit capacity of an information storage register contained therein is selectable or variable under control of control signals applied thereto.
Generally, recent large scale integrated circuits or LSIs for calculator purposes are of the combined read-only-memory (or ROM) and shift register type. If it is desired to modify the specifications of such types of LSIs, all that is necessary is to modify only ROM regions, thereby ensuring a high degree of versatility. In other words, the modifications in the LSIs specifications can be accomplished easily by controlling the thickness of a layer of SiO.sub.2 in the ROM regions without any variations in the basic layouts of the LSIs.
Nevertheless, particularly when the digit storage capacity of the shift registers should be changed, it is sometimes required to modify the layout of the LSIs. In this instance, even if allowances are previously provided for the digit capacity of the shift registers, a program sequence for the digit capacity modification should be assembled into the ROM regions. This results in a decline in the utilization efficiency of the ROM regions. These limitations on manufacture due to digit capacity modifications cause cost increases since cost reduction of LSIs is predicated largely upon mass production.